Bisphenol A [2,2-bis(4-hydroxyphenyl)propane] is usually produced by reacting phenol with acetone in the presence of a homogeneous acid or a solid acid catalyst. The reaction mixture includes unreacted acetone, unreacted phenol, water thus produced, and other by-products, in addition to bisphenol A. The main component of the by-products is 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane (hereinafter, referred to as o,p′-BPA), and in addition, it includes trisphenol, a polyphenol compound, a chroman compound, minute impurities which can cause coloring, and the like.
Examples of a homogeneous acid to be used as a catalyst, include hydrochloric acid, sulfuric acid, and the like. In the case where the homogeneous acid is used, since it is possible to proceed the reaction while precipitating crystals of an adduct of phenol with bisphenol A by reacting them at lower temperatures, bisphenol A can be produced with a high conversion of acetone and a high selectivity by decreasing the amount of the by-produced o,p′-BPA as an isomer thereof. However, the catalyst of the homogeneous acid such as hydrochloric acid requires a process for removing the catalyst from a reaction mixture or for neutralizing the catalyst, and thus the operation becomes complicated. Homogeneous dissolution of the acid in the reaction solution further causes corrosion of an apparatus or the like. Therefore, expensive and anti-corrosive materials should be used for the reaction apparatus, thus being uneconomical.
As a solid acid catalyst, a sulfonic acid-type cation-exchange resin is usually used. The reaction for producing bisphenol A essentially proceeds only with an acid catalyst, but if such a solid acid catalyst is used, the process in which acetone diffuses from the surface of the catalyst particles to an active site on the catalyst is involved, and thus gives a lower reaction rate than in the homogeneous system. Thus, there is a general method used for improving the catalytic activity and the selectivity by allowing a compound containing a mercapto group to coexist in the reaction system. Specifically, there is a method comprising charging a free-type mercapto group-containing compound such as alkylmercaptan in addition to phenol and acetone which are raw materials to a fixed-bed reactor filled with a sulfonic acid-type cation-exchange resin (for example, Patent Document 1: JP-B No. 45-10337, Patent Document 2: U.S. Pat. No. 6,414,200), and a method comprising covalently bonding a part of sulfonic acid group in a sulfonic acid-type cation-exchange resin with a mercapto group-containing compound or ionically bonding a part of sulfonic acid group in a sulfonic acid-type cation-exchange resin with a mercapto group-containing compound (for example, Patent Document 3: JP-B No. 46-19953). The method of charging a free-type mercapto group-containing compound such as alkylmercaptan in addition to phenol and acetone which are raw materials to a fixed-bed reactor filled with a sulfonic acid-type cation-exchange resin allows specific amount of mercapto group-containing compound to be existed in a reaction system at all times, and thus gives the advantage of less catalyst degradation. However, there is a concern that the mercapto group-containing compound may cause a coloring of bisphenol A, and thus requires a process for removing and recovering the mercapto group-containing compound.
On the other hand, the method of bonding a part of sulfonic acid group in a sulfonic acid-type cation-exchange resin with a mercapto group-containing compound gives a smaller loss of mercapto group-containing compound as compared to the method allowing the free-type mercapto group-containing compound to be existed in a reaction system, and thus is advantageous since there is no need of recovering the mercapto group-containing compound. In particular, there is disclosed in JP-A No. 57-35533 (use of pyridylethanethiol as a mercapto group-containing compound, Patent Document 4), JP-A No. 08-187436 (use of N,N,di-substituted mercaptoalkylamine as a mercapto group-containing compound, Patent Document 5), JP-A No. 08-089819 (use of N,N,N-trimethyl mercaptopropyl ammonium as a mercapto group-containing compound, Patent Document 6), JP-A No. 10-211433 (use of 1,4-dimercaptoalkylpiperidine as a mercapto group-containing compound, Patent Document 7), and U.S. Pat. No. 6,414,200 (use of a silicon-containing alkylmercapto compound as a mercapto group-containing compound, Patent Document 2), that the reaction rate of acetone is increased by improving the structure of a mercapto group-containing compound which to be bonded to a strong-acid ion-exchange resin.
Further, there is also a report related to a sulfonic acid-type cation-exchange resin which is an acid catalyst for improving its activity which is lower than that of the above-described homogeneous acid. When the particle diameter of used sulfonic acid-type cation-exchange resin is large, the reaction materials do not sufficiently diffuse into the particles, thus a sufficient acetone conversion cannot be obtained. Accordingly, it is suggested in JP-A No. 62-178532 (Patent Document 8) to use a sulfonic acid-type cation-exchange resin in a fine particle or a fine powder having an effective diameter of 0.3 mm or less. In JP-A No. 6-340563 (Patent Document 9), the particle diameter of sulfonic acid-type cation-exchange resin to be used and the distribution degree of the particle diameter is likewise provided, and more preferred range is disclosed. Further, in JP-A No. 4-268316 (Patent Document 10) and JP-A No. 2002-253971 (Patent Document 11), methods of forming a sulfonic acid-type cation-exchange resin having a desired particle diameter, are disclosed. As stated, the particle diameter of the sulfonic acid-type cation-exchange resin is an important factor in obtaining a sufficient reaction conversion.
Various improvements on the structure of a resin product, which is the base material of a sulfonic acid-type cation-exchange resin, have been made. The sulfonic acid-type cation-exchange resin is a resin obtained by sulfonating a styrene-divinylbenzene copolymer which is obtained by radically copolymerizing styrene and divinylbenzene. The divinylbenzene in polymerization does not only prevent a polystyrene chain from dissolving in an organic solvent, but the content thereof is also an important factor in controlling the size of a pore (size of a gel micropore) within the sulfonic acid-type cation-exchange resin formed by capturing a polar solvent, or the mechanical strength of the sulfonic acid-type cation-exchange resin. In other words, a sulfonic acid-type cation-exchange resin with a low content of divinylbenzene has a high catalytic activity due to a large gel micropore, but has a low mechanical strength. In addition, if the content thereof is high, the mechanical strength increases, but the gel micropore size decreases, which causes decreased activity.
In order to improve the diffusion within the particles, there are ion-exchange resins in which the degree of crosslinking is increased as the content of divinylbenzene is increased, that are formed with a large hole referred to as a ‘macroporous’ having a particle diameter of 20 nm or more within the particles by physical treatment. However, in the case where an ion-exchange resin having this macroporous adsorbs a molecule having high polarity, such as water, a crosslinked structure tends to inhibit the bulge of particles caused by the swelling, which eventually collapses when it can no longer endure the swelling. JP-A No. 5-97741 (Patent Document 12) and JP-A No. 6-320009 (Patent Document 13) describe a method which complements the respective defects by simultaneous filling a sulfonic acid-type cation-exchange resin having a low content of divinylbenzene and a sulfonic acid-type cation-exchange resin having a high content of divinylbenzene into a reactor. Further, an improvement on a reaction conversion is reported in Nippon Steel Chemical Co., Ltd. WO 2000/00454 (Patent Document 14), which suggests a sulfonic acid-type cation-exchange resin having large gel micropores by using large molecules such as divinylbiphenyl instead of divinylbenzene.
As such, various techniques related to catalysts have been investigated, particularly for the mercapto group-containing compound, and realized that apart from the ones which are easily available such as aminoethanethiol and pyridineethanethiol, its production process requires many reaction and separation processes, and many of the operations to obtain a product with high purity are complicated. In all cases, there is room for improvement in selectivity. The development of a high selectivity catalyst is demanded. If the improvement in selectivity is attempted, it can not only reduce the load of performing a by-product recovery process in the production process, but it also reduces the material ratio of phenol/acetone without deteriorating the selectivity by increasing the reaction temperature and thus leads to a reduction in cost related to the process for recovering excess phenol. If the activity is reduced in some extent, it can be covered by increasing the size of the reactor, and thus-caused cost-up for producing bisphenol is very small. Therefore, the development of catalyst which is easy to produce and exhibits high selectivity at an equivalent conversion is demanded.